1. Field of the Invention
The present invention relates to the implantation of ions into semiconductor materials, and in particular to apparatus in which the materials into which ions are to be implanted are supported upon a spinning disk during ion implantation.
2. Description of the Prior Art
In processing semi-conductors and, in particular, in processing of crystalline wafers by ion implantation, it is necessary to be careful to avoid the effects of so-called "ion channelling". Ion channelling is a well-known phenomenon which occurs whenever the angle of the trajectory of the incoming ion beam with respect to the normal to the surface of the semiconductor upon which the ion beam impinges (i.e., the "angle of incidence") is less than a critical angle. If the angle of incidence is near this critical angle, the ion trajectories into the material are far deeper than desired, and are deeper than the trajectories obtained through normal random distribution caused by scattering through the crystal lattice. For some purposes ion channelling is desired, and for such purposes the channelling effect is intentionally used by causing the ion beam to enter the crystal lattice along the normal axis. For other purposes random distribution is desired, and this usually occurs when the angle of incidence is varied to some angle between 3 and 10 degrees off the normal axis.
In the past, ion implantation equipment utilized various techniques for permitting adjustment of the implant angle. More recently, several machines have been designed in which silicon wafers have been fixed by clamps around the perimeter of a metal disc, which is then spun and moved across the path of a stationary ion beam for ion implantation processing of the wafers. The disk is spun to a typical speed of 500-1500 revolutions per minute (rpm). The spinning of the disk moves the wafers at high speed through the ion beam in one direction. Relative movement between the wafers and the ion beam in a direction transverse to said one direction is produced either by imparting a slow scanning movement to the beam across the wafers, or by imparting a slow scanning movement to the spinning disk. One such spinning-disk arrangement is disclosed in U.S. Pat. No. 3,778,626 to Robertson. In these past arrangements, implant angle (i.e. the angle of incidence between the surface of the wafer and the beam) has only been variable by changing the angle of the mounting platen or surface upon which the wafer is mounted. This is usually done by unbolting the flat disk-shaped surfaces which had provided zero-degree or normal incidence, and bolting them to an angled disk platen which slants the wafer mounted thereon up to some implant angle, typically 7 degrees.
Another method which has been used is to tilt the entire spinning disk with respect to the beam. This is usually done by tilting the chamber to which the disk is attached.
Another matter which must be considered when changing the implant angle is the effect of such a change on the beam measurement system if the ion beam measurement system is integral with the disk. In such an arrangement, in order to provide variable implant angle during the implant process, one must accommodate both the ability to change the angle of incidence of the beam upon the wafer as well as the ability to change the position or maintain the relative position between the dose measuring system and the disk.